1. Field of the Invention
The present invention relates to a method for determining the cut-off wavelength of an optical fibre. The present invention furthermore relates to a device for carrying out such a method.
2. Description of the Related Art
European patent application No. 0 172 328 relates to a standard method and equipment for measuring the cut-off wavelength in optical fibres, wherein a spectral scanning is effected, in particular under three different perturbation conditions. Such spectral scanning is in particular carried out at a plurality of different wavelengths with differently bent fibre positions, as is shown in FIG. 3 of the aforesaid European application.
In the article “Characterization of the bending sensitivity of fibers by the MAC-value”, Unger C., et al., Optics Communications, 107 (1994), 361–364, Elsevier, it is indicated that the MAC-value, viz. the ratio between the mode field diameter and the cut-off wavelength, is a measure for characterizing the bending sensitivity of so-called step index fibres.
From U.S. Pat. No. 5,130,535 there is known a method and apparatus for determining the bend loss along a length of optical fibre, wherein the backscattering power is measured as a function of distance along the fibre in one direction along the length of fibre by launching light energy into one end of the fibre, after which the same measurement is carried out at the other end of the optical fibre. Subsequently, a mathematical calculation is carried out on the two signals to provide a quantity which is related to mode field diameter, wherein variations in the quantity are stated to be indicative of the variations in the bend loss along the length of fibre.
From U.S. Pat. No. 6,456,370 there is known a method for measuring the cut-off wavelength of an optical fibre with an optical time domain reflectometer (OTDR) or backscattering apparatus, which method is based on the measurement of the splice loss between the fibre before bending and the optical fibre under test. More detailed information with regard to the relation between said splice loss and the cut-off wavelength is not provided. Furthermore, said US patent employs two OTDR splice loss measurements, which are always carried out in one and the same direction. Moreover, no further information is provided as regards the device that is used for introducing bends into the optical fibre.
Various characteristics of the glass fibre play an important part in the design of optical telecommunications systems comprising glass fibres. Thus, the distance to be bridged is to a large extent determined by the attenuation of the glass fibre and the dispersion characteristics. In addition to that, also the mode field diameter and the cut-off wavelength are of importance for single mode fibres. The mode field diameter is a measure of the cross-section of the optical signal in the glass fibre. For a network consisting of a chain of several fibre routes, single mode fibres having the same mode field diameter should be selected as much as possible, since otherwise the attenuation losses that occur at the connections between the various fibres will become too great.
The cut-off wavelength is a measure of the wavelength above which the glass fibre is in the single mode region. A proper guidance in the fibre can only take place for one mode, the fundamental mode, which is the basis for the very good bandwidth characteristics of the fibre. In the case of wavelengths lower than the cut-off wavelength, optical guidance through the fibre is also possible for other modes, “higher-order modes”. However, the fact that said modes have different propagation velocities leads to “multimode” behaviour, with greater signal distortion, as a result of which the desired communication characteristics cannot be obtained.
Consequently, it is of great importance for the production and the delivery of single mode glass fibre cables to ensure for every fibre that the cut-off wavelength is lower than an indicated maximum value. If the value becomes too low, however, negatives side effects are observed, such as undue attenuation of the fundamental mode in the case of light bending of the fibre, which bending especially takes place upon cabling in practice. As a result of all this, the measurement of the cut-off wavelength is an important item in the production of glass fibres intended for the glass fibre cable industry. Central elements in the internationally standardised measuring method that is used for that purpose are the fact that a spectral measurement is carried out at a series of wavelengths around the cut-off wavelength, and that the fibre has a standardised, bent position that simulates the situation in a standardised cable. The above two elements make the measurements relatively time-consuming and sensitive to the correct position of the fibre, as a result of which the measurement is furthermore slightly inaccurate in production situations. In addition, relatively costly equipment which can only be used for this purpose is required.